Code 1L indicates a "deemed distribution." That means the loan, or part of it, is being treated as a distribution in addition to being a loan. (In other words, you have to pay tax on it AND repay the loan.)
A deemed distribution occurs if the loan violates IRS regulations. The most common reasons for loan failures is the loan is too large, or the repayment schedule does not follow the appropriate rules. As a consequence for failing to follow the rules, the loan fails as a loan and is considered a taxable distribution. There is more information here.
https://www.irs.gov/retirement-plans/fixing-common-plan-mistakes-plan-loan-failures-and-deemed-distr...
You need to contact the plan, and find out why they characterized your loan as a deemed distribution. If it is a mistake they need to issue a corrected 1099-R. If you or the plan really did break the rules, you have to deal with the tax consequences. @dmertz can probably explain better than me if you have any other questions.
